This invention relates to the transfer of a paper web from the forming section to the press section in a papermaking machine. More particularly, this invention relates to the transfer of a paper web from a forming fabric to a pick-up felt in conjunction with further dewatering of the web. Still more particularly, this invention relates to the transfer of a paper web from a forming fabric over a vacuum zone on a couch roll which is effected by the nipping engagement of a pipe-up felt against the web by a transfer roll which is also equipped with a vacuum zone.
In modern papermaking machines, the transfer of the paper web from the forming section to the press section must always be effected positively, that is, the paper web must always be carried on the surface of a forming fabric or a pick-up fabric, in order to prevent web breakage at high papermaking speeds. Prior web pick-up arrangements have utilized bringing a pick-up felt into engagement with the web over a span of the forming fabric between a couch roll and a fabric turning roll. In fourdrinier-type forming arrangements, the couch roll defines the end of the generally horizontal forming section at which point the fabric is directed downwardly with the web on its surface, and it is in this downwardly extending portion of fabric travel that the transfer is effected. A typical such arrangement is shown in Wicks et al, U.S. Pat. No. 4,483,745.
It has also been common practice to pick the freshly formed paper web off the forming fabric by bringing a pick-up fabric, such as a felt, into nipping engagement with the web over a roll within the forming fabric, such as the couch roll or the fabric turning roll. Such an arrangement is shown in Wahlstrom et al, U.S. Pat. No. 3,671,389.
These arrangements are adequate and work well at papermaking speeds below about 1100 meters per minute, or with adequately dried webs capable of withstanding a relatively high vacuum pressure in the transfer roll, or with relatively heavy paper webs, or with some combination of all of these parameters.
In many modern papermaking machines, the web is formed between two, co-running forming fabrics which produce superior paper formation and more uniform fines and fillers distribution which provides a higher quality paper product. One sheet characteristic which has not always improved in such two-fabric paper web forming apparatus is the dryness of the web leaving the forming section and guided into the press section. The more relatively wet the paper web is, the greater the chance of its breaking, especially at the critical point of its removal from the forming fabric and transfer into the press section.